we know that
If 20 men can survive for 24 days on 15 cans of rations
by proportion
24/15=1/x
x=15/24----> x=0.625 cans
that means 0.625 cans for 20 men per day
20 men----------> 0.625 cans
1 man-----------> x
x=0.625/20-----> x=0.03125 cans for 1 man per day
a) how many cans will be needed for 16 men to survive for 36 days?
16*36*0.03125=18 cans
the answer is
18 can